This invention relates to membranes formed of polysulfone copolymer compositions having polymer segments covalently bonded to each other and to a process for producing membranes. More particularly, this invention relates to membranes formed of polysulfone copolymer composition having polymer segments covalently bonded to each other wherein the copolymer is present throughout the membrane solid matrix.
Porous polymer structures are generally classified according to their effective pore size or according to their retentivity i.e., the sizes of particles that are not able to pass through the pores of the porous polymeric structure. Thus, for example, the structures used as filters are classified as ultrafilters if they retain dissolved matter such as ions, proteins viruses or macromolecules, while they are classified as microporous structures if they pass dissolved matter and retain only undissolved particles. The dividing line between microporous structures and ultrafilters in terms of pore size is not defined clearly but ultrafilters generally are agreed to have an average pore size between about 0.005 and about 0.05 micrometers. A microporous structure typically has an average pore size between about 0.05 and about 10 micrometers.
Porous membranes can be described as a pore volume and a solid matrix separated by a pore wall. Porous polymeric structures used as membrane filters have also been classified according to a pore size difference at their two surfaces, i.e., as isotropic or symmetric when the two surfaces have similar pore size, and anisotropic or asymmetric when the two surfaces have different pore sizes. Porous polymeric structures have also been classified as hydrophilic or hydrophobic. When the hydrophilic structures are brought into contact with water, they will spontaneously wet, i.e. water will displace the air from the pores of the structure without the application of any external force. On the other hand, a positive pressure is required to intrude water into the pores of hydrophobic structures to displace the air.
Aromatic polysulfone (polysulfone, polyethersulfone or polyphenysulfone) membranes are widely used because of their physical robustness, and chemical and thermal stability. Along with these advantages, polysulfone membranes have certain disadvantages, related to their surface properties. The hydrophobic nature of polysulfones causes loss of permeation properties when these membranes are used to filter protein solutions, colloidal latex paints rinses, or in concentration of waste cutting oils. Also, in some applications, it has been demonstrated that a porous membrane with a charged surface will have better performance than an uncharged polysulfone membrane. Thus, a modified polysulfone that overcomes this inherent hydrophobic property of polysulfones will increase the scope of application and use of these membranes.
Much effort has gone into modifying the pore surface of pre-formed membranes. When macromolecular substances are used, this can result in change of pore properties, particularly a loss of flow, particularly for membranes having small average pore size. When heterogeneous chemical modification is used, a complicated reaction scheme is required, which can change membrane structure and properties such as mechanical strength and solubility. Furthermore, these schemes increase the complexity of the manufacturing process.
Codissolved pairs of polymers are sometimes used to change the properties of the final membrane, but this approach is limited to soluble compatible pairs.
Block copolymers are useful because the properties of the blocks of each polymer are maintained. It is common for a block copolymer to have two glass transition temperatures, corresponding to the glass transition temperature of each polymer block. Furthermore, block copolymers can be made of incompatible polymers and still have useful, indeed even improved properties, while blends of the same polymers would have poor properties, if they could even be made. Since the blocks of one polymer are covalently linked to the other polymer block, phase separation occurs only on the size level of the polymer chain segments and the deleterious effects of phase separation into relatively large domains is avoided.
Prior methods of making various types of microporous polysulfones membranes are described in U.S. Pat. Nos. 4,629,563 and 4,900,449. U.S. Pat. No. 4,629,563 describes highly asymmetric polysulfone microporous membranes. U.S. Pat. No. 4,900,449 describes symmetric porous membranes made of blends of polyethersulfone and a hydrophilic polymer to form a hydrophilic membrane. U.S. Pat. No. 5,076,925 describes microporous membranes made from a blend of polyethersulfone and a hydrophilic polymer to form a hydrophilic polymer. These membranes are hydrophilic as formed, but a post-treatment procedure allows removal of the hydrophilic polymer to render the membranes hydrophobic.
At the present time, polysulfone grafted membranes are prepared by modifying the pore wall surface of a solid polysulfone membrane with a polymerizable monomer to form a graft only on the membrane pore wall surface. The remaining portion of the membrane comprises unmodified polysulfone polymer. U.S. Pat. No. 5,468,390, Journal of Membrane Science, 105(1995), p. 237-247 and Journal of Membrane Science, 105 (1995, p. 249-259) disclose modified aryl polysulfone membranes having a hydrophilic vinyl monomer chemically grafted to their pore wall surfaces. An unmodified membrane is contacted with a solution of the monomer and is exposed to ultraviolet light to effect photochemical grafting in the absence of a sensitizer or a free radical initiator. The monomers utilized function to render only the polysulfone membrane pore wall surface hydrophilic. The remaining portion of the membrane solid matrix comprises unmodified polysulfone. These surface-modified membranes are not rewettable after they have been dried and, if dried, lose significant permeability. Therefore it is necessary to maintain the membrane surfaces wet prior to use.
Japanese Pat. No. JP-A-2-59029, published Feb. 28, 1990, discloses a process for modifying a polysulfone porous membrane on its pore wall surface only with a polymerizable monomer by immersing the membrane in the monomer solution and irradiating the solution with ultraviolet light. The process is conducted under conditions such that any solvent used in the process does not dissolve the polysulfone membrane. As a result of the process, only the pore wall surface of the porous membrane is modified to render it hydrophilic when hydrophilic polymerizable monomers are utilized in the process. As is the case with the modified polymers disclosed in U.S. Pat. No. 5,468,390, the modified membrane surfaces cannot be dried without a serious loss of permeability.
U.S. Pat. No. 5,480,554 claims a polysulfone membrane that can be dried without loss of retention properties. This membrane is hydrophobic and requires an alcohol treatment to maintain ultrafiltration properties.
Accordingly, it would be desirable to provide modified polysulfone membranes which are modified through out their solid matrix thickness. In addition, it would be desirable to provide such modified polysulfone membranes which can be dried without adversely affecting membrane permeability. Furthermore, it would be desirable to provide such a membrane which is wettable directly with water without the need for humectants such as glycerine or an alcohol treatment after being dried. In addition, it would be desirable to provide a process for forming such membranes which can be modified with a wide variety of polymerizable monomers to produce membranes formed of a polysulfone copolymer composition having polymer segments covalently bonded.